The present disclosure relates to scanning optical apparatus and image forming apparatus having the scanning optical apparatus.
The electro-photographic type of image forming apparatus like the printer and the copying machine forms a latent image on the photoreceptor (photoconductor durum) by scanning the light beams. Such light beam scanning is carried out by the scanning optical apparatus. The scanning optical apparatus is provided with laser diode (LD) as a light source, collimator lenses, cylindrical lenses, a rotary polyhedron (polygon mirror), and f-θ lenses. In the scanning optical apparatus, the emitted light beam from the light source being modulated based on an image to be formed is deflected by the rotary polyhedron, and the deflected light beam scans on the photoconductor drum in the main scanning direction. The rotary polyhedron is provided with a plurality of surfaces (five surfaces in case of pentagonal prism shape) for reflecting the light beam and a rotation axis, and the rotation axis is driven to rotate toward one direction by a driving motor.
The above-mentioned scanning optical apparatus uses plastic lenses to the scanning optical system, such as the f-θ lens penetrating the light beams deflected by the rotary polyhedron, in order to down-size and reduce cost. In case of using the plastic lens, the index of reflection varies as well as the plastic lens expands with heat to change its shape because of the change of the environmental temperature and the temperature change in the device. For instance, even if the temperature in the scanning optical apparatus is 20° C. before starting the continuous paper printing, the temperature sometimes happens to increase up to 50° C. at the end of the continuous paper printing. Since the scanning optical apparatus is accommodated in a housing made of plastic, the installation positions of the optical components change a very little due to the thermal expansion of the housing caused by the temperature change. As a result, it occurs that the scan starting point and the scan end point for the image change on the photoconductor drum, and the picture quality of the image is deteriorated.
As countermeasures for the above-mentioned problems, it is known as a conventional art that detecting sensors (BD sensors) for detecting the light beam are arranged at a specific position before the san start and at a specific position after the scan end respectively. In the conventional art, the magnification errors in the main scanning direction is corrected by adjusting the scanning time (between the scan start time and the scan end time) according to the time interval of the light beams detected by two sensors.
There is a configuration that the scanning optical apparatus is not provided with the reflective mirror and the photo diode (BD sensor) for generating the scan-start signal in order to reduce the number of components and simplify the assembling adjusting operation. In such configuration, the light emitted from a laser oscillator that is a power source is reflected by the scanning mirror and then returns to the laser oscillator, and the returned light is detected to be the scan start signal. The returned light is detected by a disorder signal generated in the driving current of the laser oscillator at receiving the light.